Radiation therapy (also referred to as radiotherapy) may be used in the treatment of cancer or other pathologies. Radiotherapy involves delivering a prescribed dose of radiation to a target region of a patient, for example, to a tumor or other cancerous tissue. The target region may be imaged prior to the administration of radiotherapy, and a treatment plan may be formulated based on, e.g., the size, location, and/or orientation of the target and the surrounding structures, among other things. A linear accelerator (linac) may then be used to deliver radiation to the target region of the patient. The linac may direct photons (e.g., an X-ray), electrons, or other subatomic particles toward a target, such as a tumor.
After initial images of the target are acquired, however, the location and/or orientation of the target region may change. For example, the patient may shift during transfer to the treatment room, during movement within the treatment room (e.g., positioning on a couch, bed, or table), or during the administration of radiotherapy. For example, a patient may move voluntarily or involuntarily due to regular biological processes, including, e.g., breathing, swallowing, blinking, twitching, peristalsis, digestion, beating of the heart, coughing, passing gas, or other movements.
Changes in the location and/or orientation of the target region may reduce the efficacy of radiotherapy. For example, if the actual orientation or location of the target region is different than the assumed orientation or location based on prior imaging, then the correct dose of radiation may not be delivered to the intended target region. Additionally, surrounding healthy structures may receive radiation instead of, or in addition to, the intended target region. Exposing the wrong area to radiation may ultimately harm or kill surrounding healthy cells. Accordingly, accurate, real-time, 3D localization and tracking of a target may be desirable during radiotherapy to account for movement (e.g., movement of a tumor or movement of surrounding healthy structures) as radiation is delivered to the patient.
Medical imaging may be used to control for and accommodate changes in the location and/or orientation of a target region after the acquisition of initial imaging. Imaging systems, including, for example, CT, cone-beam CT (CBCT), fluoroscopy, X-ray, and/or MRI may be used before and/or during the delivery of radiotherapy to determine the location of and track a target region. Such imaging systems may be incorporated into radiotherapy delivery systems, for example, into an image-guided linac, to enable gating or tracking strategies to compensate for movement of the target region in real time during the delivery of radiotherapy. Such technology may be referred to as image-guided radiation therapy (IGRT) or intensity modulated radiation therapy (IMRT).
Currently available technology, however, has struggled to produce accurate, real-time localization of a target region and/or surrounding structures. Conventional linear accelerators may include a kilovoltage (kV) imager affixed to a gantry, enabling imaging perpendicular to a megavoltage (MV) treatment beam. A kV imager may be used to acquire 2D X-ray projections at any given point in time as the imager moves around the patient on the gantry.
Although kV projection images alone are useful in some instances, particularly for high-contrast targets or patients with embedded fiducials, it is often desirable to acquire multiple projections from multiple viewpoints. For example, the X-ray imager may be rotated in an arc around the patient (e.g., along a gantry) to acquire new projection images at angular increments. A 3D image may then be reconstructed from multiple projections using principles of tomography.
Yet, the 3D images reconstructed using currently available technology generally are not able to accurately depict the location and orientation of a target area in real time. This is because as an imager moves along the gantry to capture images of the target region from different angles, only the current projection image is accurate—all of the previously acquired projection images may be stale and no longer depict the current location of the target region. While the stale images are needed to reconstruct a 3D image, the stale images may contain incorrect location data. Only the current projection indicates the true location and orientation of the target region at that time, thus averaging the current image with the stale images may decrease the accuracy of the resulting image. Attempts have been made to combine current and stale images using algorithms and interpolation, but many of these techniques have struggled with inaccuracies. The unique cone-beam shape of CBCT complicates the application of many algorithms, and performing these algorithms in the spatial domain has proved unwieldy because of the amount of data that must be computed so quickly. In some instances, the algorithms used have been too computationally complex for fast implementation on 3D data and thus are not useful for real-time motion management. The construction of real-time (3D+T) CBCT images has been referred to in the literature as ‘cine CBCT’.
As an alternative solution to detecting real-time motion during treatment, attempts have been made to detect the target directly in each individual projection. The target may then be known to exist along a ray line connecting the detected image pixel and the target source. If stereoscopic kV imaging is used (e.g., Cyberknife technology), then the target position may be determined by intersecting ray lines from each detector. If a single kV detector is present, as is the case with many modern linacs, then monoscopic kV imaging techniques may be used to estimate the position of the target along the ray line. Yet, such techniques may result in the loss of information regarding the full target and surrounding tissues. They also rely on being able to detect the target in each kV projection, but kV imaging may generally only be effective for imaging high-contrast targets, for example, with the use of implanted fiducials, which limits the applicability of such techniques. Such attempts have been termed ‘cine projection’ solutions. With a ‘cine CBCT’ rather than a ‘cine projection’ solution, lower contrast targets may be detected, often without the need for fiducials, but again, the computational power necessary to perform such calculations may not be feasible for use with real-time applications.
Accordingly, a need exists for systems and methods that allow for the generation of accurate, real-time images of a target region that allow a healthcare provider to track the location and/or orientation of the target region in a patient before, during, and/or after the administration of radiotherapy. There also exists a need for systems and methods of tracking movement of lower contrast targets and for tracking movement of targets without using fiducials.